This invention relates to a method of manufacturing resin tubes alternately having a thick wall portion and a thin wall portion.
Generally, an extrusion molding process is employed to manufacture resin tubes. This extrusion molding process comprises providing an orifice at the tip of an extruding machine, forming a tubular opening at the extreme end of the orifice, extruding resin through said opening to form it into a tubular member, and pulling or taking out the molded tubular resin to manufacture a resin tube.
Conventionally there is employed an extruding machine of the type in which one or more screws are rotated within a barrel and the molten resin is mixed while molten by the screw which is designed so as to extrude the resin at a substantially uniform speed. In such an extruding machine there is used an orifice in which an opening width is formed to be substantially uniform along the axis of flow in order to provide a contour as desired. It is therefore difficult for such apparatus to manufacture resin tubes having a thick wall portion and a thin wall portion.
Extruding machines may extrude resin at substantially a uniform speed, but strictly speaking, the extrusion speed of the extruding machines is not always constant due to various causes. In normal extrusion operation, therefore, the pulling or take-up speed has not always been made constant but has been varied responsive to variation of extrusion speeds. Taking advantage of the fact as noted above, it may readily be conceived that in normal extruding operation, the take-up speed is periodically varied to thereby alternately form a thick wall portion and a thin wall portion along the axial direction of a tube.
Japanese Patent Publication No. 24143/1974 describes an attempt to manufacture resin tubes with thick and thin wall portions in accordance with the above mentioned principles. However, satisfactory resin tubes have not been obtained with the use of orifices as disclosed in FIGS. 1 and 2 of said patent. The orifice shown in FIGS. 1 and 2 of said patent is formed with an opening corresponding to the thin wall portion of the resin tube, and the resin tube emerged from the orifice is externally controlled in its outside diameter. However the system of the latter patent results in buckling of the walls of the tube. As the tube is pulled it passes immediately into an outside diameter control device which acts to harden the external surface. However as a result of the reactive force in the axial direction caused by the pressure of the resin extruder and the force in the opposite direction caused by slowing the pulling speed, the tube will buckle. Also the physical properties of the tube are degraded by residual strain due to the different flowing speeds of the resin at the internal and external surfaces which are caused by the quick hardening of the external surface. For this reason, further improvements are necessary to manufacture resin tubes of the aforementioned kind by conventional extruding machines are described above.